Silicone hydrogel contact lenses can be manufactured economically in large numbers by a conventional full-mold process involving disposable molds. In a conventional molding process, a predetermined amount of a polymerizable material typically is introduced into a disposable mold comprising a female (concave) mold half and a male (convex) mold half. The female and male mold halves cooperate with each other to form a mold cavity having a desired geometry for a contact lens. Normally, a surplus of polymerizable material is used so that when the male and female halves of the mold are closed, the excess amount of the material is expelled out into an overflow area adjacent to the mold cavity. The polymerizable material remaining within the mold is polymerized by means of actinic radiation (e.g., UV irradiation, ionized radiation, microwave irradiation) or by means of heating. The starting material in the mold cavity is cured to form a lens while the excess material in the overflow area is partially or completely cured to form flashes. After curing, the mold is separated into the male and female mold halves with the formed lens adhered onto either male or female mold half.
A various mold materials can be used to make disposable molds for contact lenses (for example, U.S. Pat. Nos. 5,843,346, 5,849,209, 5,975,875, 5,965,630, 6,551,531, 6,638,451, and 6,821,608, and U.S. Published Patent Application No. 2007/0036878A1). However, various problems may exist when using these known mold materials for making mold for the product of silicone hydrogel contact lenses. First, some mold materials may have relatively large non uniform anisotropic shrinkage over time after injection molding, causing dimensional changes in molds and large fluctuations in the parameters (peak refractive index, diameter, basic curve, central thickness etc.) of contact lenses produced from the molds to be produced.
Second, some mold materials have inadequate chemical resistance to silicone hydrogel lens formulation. Typically, a silicone hydrogel lens formulation comprises one or more organic solvent and hydrophilic and hydrophobic vinylic monomers. The organic solvent and vinylic monomers, alone or in combination, may be able to chemically and/or physically etch away some part of the mold surfaces of molds. As such, the resultant lenses made from those molds may have damaged lens surfaces.
Third, some mold materials have high oxygen permeability. Cast molding of silicone hydrogel contact lenses using molds made from those mold materials with high oxygen permeability may have to carried out under oxygen-free atmosphere or under a stringent control to minimize the adverse effect of oxygen on the ion permeability of resultant lenses. It is known that on-eye movement of a contact lens is required to ensure good tear exchange, and ultimately, to ensure good corneal health. Ion permeability is one of the predictors of on-eye movement, because the ion permeability is believed to be directly proportional to the permeability of water. High ion permeability is one of desirable properties of a silicone hydrogl contact lens. When a mold made of a material with high oxygen permeability is used to cast-mold-silicone hydrogel lenses, the resultant lenses may have low ion permeability.
Fourth, some mold materials may not have adequate UV transmissibility when the curing of a lens formulation in a mold is carried out by UV irradiation.
Because the known lens mold materials can have one or more problems discussed above, there still exists a need in the art for mold materials suitable for cast-molding of silicone hydrogel contact lenses with high quality and an improved method of cast molding silicone hydrogel contact lenses with molds formed from these mold materials.